Stretching corundum crystals



1949; E. McCANDLESS ETAL 2, 7

I STRETCHING CORUNDUH CRYSTAL I Filed May 17, 1944 rv/. r I. v G l/ I//l INVENTOR 5 LESS DONALD M. YENNI EDWARD L. McCAND ATTORNEY PatentedOct.- 25, 1949 I STRETCHING CORUNDUM CRYSTALS Edward L. McCandless andDonald M. Yenni, Kenmore, N. Y., assignors to The Linde Air ProductsCompany, a corporation of Ohio Application May 1'7, 1944, Serial No.536,058

3 Claims.

- This invention relates to hot stretching unicrystalline precious andsemiprecious stone bodies composed of corundum and, more particularly,to hot stretching thin rods of corundum. The invention is also concernedwith novel stretched unicrystalline corundum articles of manufacture.

Corundum crystals, both natural and synthetic, have long been used forjewel bearings, and as gems, heretofore having been formed into variousshapes by mechanically cutting, grinding, and polishing them. Suchmechanical operations are slow, tedious, and expensive. Furthermore, thenumber of shapes which may be produced is strictly limited to thosehaving a relatively simple contour.

The principal object of the present invention is the provision of anovel method of hot forming unicrystalline precious stone bodiescomposed of corundum by stretching them. Another object is the provisionof a novel method for stretching a corundum body to form a neck portionof circular cross section.

Another object of the invention is the provision of a stretched corundumrod having a neck of circular cross section.

The above and other objects, and the novel features of the invention,will become apparent from the following description, having reference tothe figures of the accompanying drawing, wherein:

Fig. 1 is a vertical sectional view of a corundum rod illustrating themeaning of optical orientation with respect to the C-axls;

Fig. 2 is an elevational view showing a stretched corundum rod;

Fig. 3 is an enlarged cross-sectional view taken along the line 3-4 ofFig. 2; and

Fig. 4 is an enlarged cross-sectional view through the stretched portionof a corundum rod which has been reheated after stretching.

Unicrystalline synthetic corundum bodies are composed predominantly ofalumina, with which sometimes are incorporated small amounts of coloringmaterials such as chromium oxide for rubies, or iron oxide and titaniumdioxide for blue sapphires. Corundum is grown synthetically as largediameter boules, and as long substantially cylindrical thin rods by thewell-known method of passing alumina powder through a gas flame andaccumulating the resulting fused alumina on a suitable support.

In its broader aspects, our novel method of hot forming single crystalsof corundum by deforming them comprises heating to its softeningtemperature at least a portion of such a crystal.

and deforming the heated portion, which is in a state of plasticity, byapplying force thereto. Best results can be obtained by selecting acrystal of known and controlled orientation for hot forming. Theoperator can recognize when the moment to apply force has arrived by thecolor of the crystal and by its resistance to deformation whenexploratory efforts at deformation are made. Deformation is generallyaccomplished when the crystal is at a readily recognizable white heat.Actual temperature measurements with an optical pyrometer have shownthat the temperature necessary for stretching varies with the roddiameter. However, for corundum rod, the minimum temperature formanually stretching is about 1700 F. Corundum rods of 0.090 inchdiameter require a temperature of about 2400 F., and 0.130 inch rodsrequire a temperature of about 2530 F. for manual deformation. Thesetemperatures are considered accurate to '-100 F.

When synthetic corundum bodies are hot formed by deforming them in themanner described briefly above, the heated portions also are usuallysimultaneously glossed by the heat of the flame, which seems to meltsuperficially the crystallites on the surface of the body and cause themelted portions to coalesce into a smooth and glossy surface. It is notcertain that heat glossing actually occurs in the described way, as itmay occur by solid diffusion. It is possible, however. to hot form thebodies at temperatures so low that no heat glossing occurs.

Corundum crystallizes in the hexagonal system and has a. singleprincipal optic or C-axis, represened on the rod I l of Fig. 1 by theline C-C, which lies in the direction in which light may be passedthrough the crystal without being doubly refracted. used with referenceto corundum in this application, refers to the angle 0 included betweenthe C-axis and the longitudinal or growth axis G-G of the syntheticcorundum rod as shown in Fig.

It has been found that an important relation exists between theorientation of corundum bodies and their behavior in hot formingoperations. Since the optic orientation in the deformed portion of acorundum rod changes during deformation, the resulting product has theoriginal orientation in the undeformed portion or portions, but thedeformed portion has an orientation differing from the adjacentundeformed portion or portions. However, the resultant products havesubstantially continuous crystallographic propertles throughout, i. e.the change in The term optic orientation," when assume 3crystallographic properties through the deforme portion is gradualrather than abrupt.

Heating of unicrystalline corundum bodies for deformation is ordinarilyaccomplishedin a gas flame projected from a standard blowpipe, similarto a glass blower's'torch; A suitable flame is desirably formed byburning a mixture of oxygen, natural gas, and air, but any othersuitable.

combination of fuel gas and oxygen may be used. The flame itself shouldbe broad enough so that at least one inch of the material to be workedcan be heated to its plastic temperature. It is advantageous to heat thesingle crystals in a gas flame of the type described, but it is apparentthat they may be heated in other ways, such as elasticity andresilience, as well as good resistance to chemical action.

Products fashioned from corundum by the method of the invention haveundiminished resistance to chemical corrosion, refractoriness at hightemperatures, mechanical strength, and hardness. Furthermore, suchproducts are mechanically stronger than similar products which have beenmechanically cut or ground from solid gem bodies, because the highlyflame-glossed surfaces' existing on the finished products eliminate asstretching continues, as shown at is on the rod 2| of Figs. 2 and '3. Bycutting oil the unstretched end portions and 21, a corundum ribbon isobtained. If stretching is continuous, the neck will break before anygreat extension is obtained. It has been found, surprisingly, that thiselliptical ribbon-likeneck assumes a symmetrical shape of substantiallycircular crosssection when reheated in the flame, as shown at 23 in Fig.4. Corundum can be stretched into the shape of a long thin hair-likefilament by repeatedly stretching the rod a relatively small amount toform a fiat ribbon, reheatin to provide a symmetrical cross-section, andthen stretching again until a filament of the desired thinness andlength is obtained.

Clear corundum rods (white sapphire) become milky and opaque in thestretched portion while remaining clear and non-milky in the unstretchedend portions. According to one theory, this is due to the breakdown ofthe crystal lattice into fine crystal blocks having nearly parallelsides.

It has been found most advantageous to stretch corundum rods wherein theoptic orientation is between 20 degrees and 60 degrees. If a rod hav-ving an initially high optic orientation is stretched,

only a very small amount of stretching -is possible because the opticorientation in the stretched neck portion increases with stretching andmay rise above the upper limit of 60 degrees. With rod sections havingoptic orientations below 20 degrees it is diflicult to apply suflicienttension so that the component of the shear stress in the basal plane ofthe crystal is sufiicient to start the stretching.

Rods which have been stretched in this manner are useful as threadtensioning posts and thread guides in textile mills because of theirhigh raistance to the abrasive action of the.

thread, and because the hollow formed by the neck acts as a groovewithin which the thread may run. The heat resistance and mechanicalstrength of filaments of corundum and spinel also are so great that suchfilaments would be useful as supporting elements in vacuum tubes.Moreover, the thin filaments can be coiled while hot to form smalldiameter springs having good the notch effect due to microscopicscratches on the' surfaces of mechanically worked bodies. Articles hotformed from corundum by the stretching process described herein are.more rapidly and less expensively fabricated than would be possible bymechanical cutting, grinding, and polishing operations.

What is claimed is:

1. A method comprising heating to plasticity at least'a portion of aunicrystalline corundum rod; stretching such portion by applying tensionthereto, thereby forming a ribbon-like neck; and rendering said necksubstantially circular in cross-section by reheating said neck.

2. A method for stretching a thin unicrystal line rod of corundum to agreater extent than can be accomplished in one continuous stretchingoperation, which method comprises heating a portion of such a rod toplasticity; stretching such portion by applying tension thereto, therebyforming a neck having an elliptical crosssection; converting said neckto a circular crosssection by reheating said neck; and then stretchingsaid neck to a greater length while heated to plasticity.

3. A unicrystalline corundum rod having an optic orientation between 20and degrees. said rod having two clear non-milky spaced sections ofrelatively large diameter and a neck of reduced diameter and circularcross-section be-' tween said sections and merging smoothly therewith,said neck having a glossy scratch-free surface and a milky appearance.

EDWARD L. McCANDLESS. DONALD M. YENNI.

REFERENCES CITED The following references are of record in the file ofthis patent:

, UNITED STATES PATENTS Synthetic sapphire production reaches commercialscale in U. 8., Reprinted from Product Engineering, Oct, 1943 issue, TheLinde Air Products Co., New York.

